M87* Black Hole Study Maps Plasma Physics Using Dual-Frequency Images

Scientists Use Dual-Frequency Images to Explore Black Hole Plasma Physics

Beyond the First Image

The first-ever image of a black hole—a glowing, ring-like structure at the heart of galaxy M87—was unveiled to the world in a landmark moment in 2019. Now, researchers have taken the next significant step in black hole science. By moving beyond capturing a simple picture, the team aimed to understand the physics of the environment through dual-frequency images. The study was conducted by scientists at the Shanghai Astronomical Observatory (SHAO) of the Chinese Academy of Sciences (CAS), along with international collaborators, who combined horizon-scale images obtained in 2018 from the Event Horizon Telescope and the Global Millimeter Very Long Baseline Interferometry (VLBI) Array.

The team utilized data at two distinct frequencies: 1.3 mm and 3.5 mm. By carrying out this dual-frequency spectral study, the researchers produced a spatially resolved spectral-index map on event-horizon scales. This allowed them to determine the physical conditions of the plasma around the black hole and the specific processes that generate the observed radiation.

Mapping Plasma Physics at the Event Horizon

The results show that the radiation properties surrounding the black hole vary systematically with distance, as revealed by the spatial distribution of the spectral index. The spectral index, which is a measure of how radiation intensity changes across wavelengths, provided the team with a window into the plasma’s physical state:

• The Innermost Region: Near the black hole, the spectral index is positive and increases slightly with radius. This suggests that the dense plasma in this core zone is heavily impacted by synchrotron self-absorption, where the plasma reabsorbs its own emitted radiation.
• The Outer Region: Farther from the center, the spectral index drops and transitions into negative values. This shift indicates a transition toward a more optically thin emission regime, where radiation can more freely escape without being reabsorbed by surrounding particles.

The 30 μas Transition

A key takeaway from the study is that this physical transition occurs at a distance of about 30 microarcseconds (μas) from the black hole. This distance is consistent with the radius of the ring-like structure observed at 3.5 mm. According to the researchers, this spatial overlap suggests that the ring-like structure seen in black hole images is not merely a feature of the emission morphology, but is closely connected to the physical state of the plasma near the event horizon.

By obtaining the first spatially resolved spectral-index distribution of the M87 black hole, we can quantitatively characterize how the radiation properties change across the region surrounding the black hole, said Dr. ZHAO Shanshan, an assistant researcher at SHAO and the first author of the study. This allows us to directly explore how the plasma properties vary on horizon scales and provides new clues for understanding accretion flows and jet formation.

Future Directions for Multi-Frequency Imaging

Dr. LU Rusen, the corresponding author of the study, noted that multi-frequency imaging allows researchers to disentangle the complexities of gravity from the dynamics of the plasma. By separating these variables, scientists can better model the environments where black holes consume matter and launch powerful jets.

Looking toward future developments, the researchers stated that continued advances in millimeter VLBI will enable observations at more frequencies, with higher sensitivity and time-resolved imaging capabilities. These improvements are expected to provide even deeper insights into the physical conditions governing the regions immediately surrounding black holes.

Sources: PHYS, openaccessgovernment.org.